From: Sean Christopherson <seanjc@xxxxxxxxxx>
commit 1aa0e8b144b6474c4914439d232d15bfe883636b upstream.
Add a config option to guard (future) usage of asm_volatile_goto() that
includes "tied outputs", i.e. "+" constraints that specify both an input
and output parameter. clang-13 has a bug[1] that causes compilation of
such inline asm to fail, and KVM wants to use a "+m" constraint to
implement a uaccess form of CMPXCHG[2]. E.g. the test code fails with
<stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
^
<stdin>:1:29: error: unknown token in expression
<inline asm>:1:9: note: instantiated into assembly here
.long () - .
^
2 errors generated.
on clang-13, but passes on gcc (with appropriate asm goto support). The
bug is fixed in clang-14, but won't be backported to clang-13 as the
changes are too invasive/risky.
gcc also had a similar bug[3], fixed in gcc-11, where gcc failed to
account for its behavior of assigning two numbers to tied outputs (one
for input, one for output) when evaluating symbolic references.
[1] https://github.com/ClangBuiltLinux/linux/issues/1512
[2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@xxxxxxxxxx
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096
Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-Id: <20220202004945.2540433-2-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
init/Kconfig | 5 +++++
1 file changed, 5 insertions(+)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -68,6 +68,11 @@ config CC_HAS_ASM_GOTO_OUTPUT
depends on CC_HAS_ASM_GOTO
def_bool $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
+config CC_HAS_ASM_GOTO_TIED_OUTPUT
+ depends on CC_HAS_ASM_GOTO_OUTPUT
+ # Detect buggy gcc and clang, fixed in gcc-11 clang-14.
+ def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null)
+
config TOOLS_SUPPORT_RELR
def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)
